The present invention relates to a motor driving apparatus that drives an induction motor at variable speeds, and relates in particular to a driving apparatus that can output a high torque at the time a motor is activated.
Recently, variable-speed driving of motors using an inverter has been developed to save energy and prevent global warming, or to provide improved efficiency for production lines. FIG. 8 is a diagram showing a conventional example system for controlling an induction motor. In FIG. 8, a speed controller 11 calculates a torque instruction τ*, and a current instruction calculator 13 employs the torque instruction value τ* and a magnetic flux instruction Φ*, received from a magnetic flux instruction calculator 12, to calculate a torque current instruction Iq* (perpendicular to the magnetic flux of a motor) and an excited current instruction Id* (parallel to the magnetic flux of the motor), so that an estimated speed value ωr^ matches a speed instruction value ωr*.
That is, the speed controller 11, the flux instruction calculator 12 and the current instruction calculator 13 constitute current instruction calculation means for calculating a current instruction value based on a deviation between the speed instruction value ωr* and the estimated speed value ωr^. Since the control process is performed by dividing current elements into those along the axis (d axis) parallel to magnetic flux elements and those along the perpendicular axis (q axis), this process is also called a vector control process. A Q-axial current controller 14 and a d-axial current controller 15 calculate a q-axial voltage instruction Vq* and a d-axial voltage instruction Vd*, so that a torque current detection value IqFB=the torque current instruction Iq* and an excited current detection value IdFB=the excited current instruction Id* are established. These controllers 14 and 15 constitute current control means for controlling a current based on a current instruction value. The d-axial voltage instruction Vd* and the q-axial voltage instruction Vq* are converted into three-phase AC voltage instructions Vu*, Vv* and Vw* by employing a phase θ1, which is obtained by performing the integration of a frequency instruction value ω1* for an inverter that will be described later. Then, a PWM inverter 2, which is connected to a three-phase AC power source 1, performs PWM modulation for the three-phase instructions, and transmits the results, as the output voltage of the three-phase AC inverter, to an induction motor 3. The PWM inverter 2 performs switching by employing a semiconductor device, such as an IGBT. An induction voltage calculator 19 employs, for example, the following expressions (1) and (2) to convert the d-axial voltage instruction Vd* and the q-axial voltage instruction Vq* into motor induction voltages Ed and Eq. In these expressions, r1 denotes the primary resistance of a motor, Lσ denotes the sum of primary reduced values of leakage inductances of the motor, and P denotes a differential operator (d/dt).Ed=Vd*−r1×Id−Lσ×P×Id+ω1×Lσ×Iq  (1)Ed=Vq*−r1×Iq−Lσ×P×Iq−ω1×Lσ×Id  (2)ω1*=Eq/Φ*  (3)
The induction voltage calculator 19 and the frequency instruction calculator 20 constitute frequency instruction calculation means that employs the output voltage instruction value to calculate the frequency instruction value ω1* in the above described manner. It should be noted that the frequency instruction value ω1* may be calculated by employing a voltage detection value, instead of the voltage instruction value.
Based on expression (4), a slip calculator 17 employs the torque current instruction Iq* and the magnetic flux instruction Φ* to calculate an estimated slip speed value ωs^ for the motor. Further, in accordance with expression (5), a speed addition unit 18 calculates the estimated speed value ωr^ for the motor. It should be noted that in expression (4), T2 denotes a secondary time constant of the motor, and M denotes a mutual inductance of the motor.ωs^=1/T2×M×Iq*/Φ*  (4)ωr^=ω1*−ωs^   (5 )
In this manner, the torque instruction value τ* is determined so that the estimated speed value ωr^ matches the speed instruction value ωr*. Then, a current is controlled so as to match the excited current instruction Id* and the torque current instruction Iq*, which are determined in accordance with the torque instruction value τ*. It should be noted that the torque instruction value τ* may be provided directly as an operating instruction, instead of being obtained through calculations based on a deviation between the speed instruction value ωr* and the estimated speed value ωr^.
Further, another method is disclosed in Japanese Patent No. 3070391 (paragraph [0011]), for example, whereby, when a large torque must be generated at the time a motor is activated, a magnetic flux instruction value is raised to obtain increased torque (∝ magnetic flux of a motor×a current).